1. Field of the Invention
The present invention relates to an ink jet printing method and an ink jet printing apparatus, more particularly relates to an ink jet printing method and an ink jet printing apparatus, in each of which an ink image is formed on a printing medium by use of an intermediate transfer body.
2. Description of the Related Art
There are the following two methods of reproducing gradations of a print.
1. Area gradation: method for pseudoreproducing gradations by difference of a size of coloring area per unit area with ink density same.
2. Density gradation: method for reproducing gradations by difference of ink densities with the size of coloring area per unit area same.
Ink jet printers, electrophotography, offset printing, flexographic printing, screen printing and the like can be taken as printing systems in which the area gradations are used. Meanwhile, photogravure printing can be taken as a printing system in which the density gradation is used.
The area gradation realizes excellent resolution, and is thus advantageously used to reproduce characters and lines. However, an area of a lower ink density, which is to be colored, has a smaller size. Accordingly, an amount of exposure of the surface of a printing medium in an image increases (a larger difference in density is caused between the image on the printing medium and an original). Accordingly, graininess becomes apparent.
From the above-described characteristics, the printing system, in which the density gradation is used, is preferable since a smooth gradation reproduction can be realized, especially in the case of outputting an image such as a photo. And in addition to photoprinting, in a case where output of a smooth and high-quality image is required, an image is printed preferably by use of the density gradations.
It is desirable that the density gradation be performed to obtain high-quality photo output, as described above. However, it is difficult to realize the density gradation with the current ink jet printing technique. The density gradation is not realistic even if lighter ink is used, since the number of ink types required in different densities correspond to the number of gradations requested. In order to reproduce 32 gradations of each of four colors CMYK, 124 types of ink (4×31=124) are necessary.
By the way, the ink jet printing method in which printing is carried out by use of the intermediate transfer body has been proposed as an ink jet printing technique. In the system in which the intermediate transfer body is used, it is important to accurately form an ink image on the surface of the intermediate transfer body, and to keep the ink image without causing distortion of the image until the ink image is transferred to a printing medium. Besides, in Japanese Patent Laid-Open No. 2004-42454, a sea portion (water-repellent portion) and island portions (hydrophilic portions) are formed on the surface of the intermediate transfer body for the purpose of keeping points in each of which ink lands. The sea portion has a larger contact angle with water of ink. Each of the island portions has a contact angle smaller than that of the sea portion. The ink having landed in the island portions, each of which has a contact angle smaller than that of the sea portion, does not spread to the sea portion, and is kept within the island portions. In other words, each of the ink droplets does not spread beyond the corresponding island portion because the island portions are surrounded by the sea portion having a contact angle larger than that of each of the island portions. Accordingly, image distortion can be reduced until the ink image is transferred from the intermediate transfer body to the printing medium, and thereby the ink image can be satisfactory transferred to the printing medium.
However, there still remain problems of the above-described gradation reproducibility even in such an ink jet printing method in which the intermediate transfer body is used. Improvement needs to be made to solve the problems. Specifically, the above-described density gradation is not realized in Japanese Patent Laid-Open No. 2004-42454. This is because gradations correspond to only two values respectively indicating that “ink exists” and that “no ink exists,” although the sizes of the ink dots are regulated since the “island portions” are provided to the surface of the intermediate transfer body as portions to which the ink can be supplied.